1. Field of the Invention
The present invention relates to a display device which includes a pixel array having a plurality of pixels arranged in a matrix form and wirings, common to the plurality of pixels and disposed in two directions, intersecting with each other in the pixels. More specifically, the present invention relates to a wiring forming technique for minimizing the number of wirings disposed in the two directions and bringing the resistances thereof close to each other to the extent possible.
2. Description of the Related Art
Some display devices use electro-optical elements whose brightness changes with change in voltage applied thereto or current flowing therethrough. For example, a liquid crystal display element is a typical example of an electro-optical element whose brightness changes with change in voltage applied thereto. A display device using such elements is called a “liquid crystal display device.”
On the other hand, an organic electro-luminescence element is a typical example of an electro-optical element whose brightness changes with change in current flowing therethrough. An organic electro-luminescence element is commonly called an OLED (Organic Light Emitting Diode). A liquid crystal display element is an optical modulation element (i.e., non-self-luminous) adapted to modulate light from the light source. An OLED differs from a liquid crystal display element in that the former is a self-luminous element which can emit light by itself. A display device using OLEDs will be herein referred to as an “organic EL display device.”
Such liquid crystal display devices have become increasingly large in screen size, high in definition and low in cost, finding widespread use on a full-scale basis ahead of a complete transfer to digital broadcasting. As a result, these display devices are demanded to be further reduced in cost.
On the other hand, organic EL display devices offer many advantages over their liquid crystal counterparts, including no need for backlight, lower power consumption and possible reduction in thickness, and excellent moving image display performance. As a result, these devices are seen as a next-generation of display devices. Development efforts are moving at an accelerated pace to replace some of the liquid crystal display devices with organic EL ones with an eye toward widespread use of liquid crystal display devices on a full-scale basis.
In the development efforts for display devices typified by liquid crystal and organic EL display devices, it is essential that the manufacturing process be as simple as possible to prevent point and line defects for enhanced yield in order to provide a larger screen size. In particular, the empirical rule says that the fewer the lithography process steps adapted to perform patterning using resist, an organic material, the significantly less likely that defects will occur. Based on this rule, the wiring process is undergoing simplification (refer, for example, to a wiring structure having one aluminum layer described in Japanese Patent Laid-Open No. 2004-4192 and Japanese Patent Laid-Open No. 2004-317916 (hereinafter referred to as Patent Documents 1 and 2 regarding organic EL display device).
Both of the techniques disclosed in Patent Documents 1 and 2 offer a simpler wiring structure in the pixel using chromium layer and its overlying layer. The chromium layer is at the same hierarchical level and made of the same material as the gate electrode of the thin film transistor (TFT). The overlying layer is made of aluminum.